1. Field of the Invention
The present invention relates generally to satellite communications and, more particularly, to a method for forwarding an incoming call received in a satellite communication terminal in a high penetration notification mode to a communication terminal connected to a PSTN (Public Switched Telephone Network) or an ISDN (Integrated Services Digital Network).
2. Description of the Related Art
A satellite communication terminal has a middle penetration (MP) mode and a high penetration notification (HPN) mode. In the MP mode, a satellite communication system provides a voice service which is a basic service, and a short message service which is a supplemental service. In the HPN mode, which is set when the satellite communication terminal is in an area where MP mode service cannot be provided, the satellite communication system informs the satellite communication terminal that a call (i.e., voice service or short message) is being received by transmitting a high-power, high-penetration radio wave.
FIG. 1 is a block diagram of a general satellite communication system. A service center 40 controls the overall operations of the satellite communication system. A switching station 20 switches a call received from a first communication terminal 5 through a PSTN or ISDN 10 or from a second communication terminal 85 through another PSTN or ISDN 70. The switching station 20 has a subscriber database 22 and a subscriber authenticator 24. The subscriber database 22 stores information about satellite communication subscribers. The subscriber authenticator 24, under the control of the service center 40, checks whether or not a calling subscriber is a subscriber registered in the subscriber database 22. A satellite base station 30 receives a call switched from the switching station 20 and transmits it as a radio signal to a satellite 50. The satellite 50 receives the radio signal from the satellite base station 30 and forms a link with a satellite communication terminal 60. A reference number 80 denotes a closed space 80, such as an underground subway station or the interior of a building.
In the MP mode, the satellite communication terminal 60 can directly communicate with the satellite 50 to provide the voice service and the short message service. However, if the satellite communication terminal 60 moves into the closed space 80 where it cannot directly communicate with the satellite 50, then the services in the MP mode are not offered. This is because a radio wave sent from the satellite 50 has a property of traveling straight. Therefore, the satellite 50 transmits a high-power, high-penetration radio wave to inform the satellite communication terminal 60 within the closed space 80 that a call is being received. The satellite 50 sends both a radio wave of the MP mode and a radio wave of the HPN mode (i.e., a high power radio wave) to an HPN service subscriber. When the radio wave of the MP mode and the radio wave of the HPN mode reach the closed space 80, the radio wave of the HPN mode passes through the closed space 80.
If the satellite communication terminal 60 is situated within the closed space 80 and a call from another communication terminal is received by the satellite communication terminal 60, only the radio wave of the HPN mode is received. The radio wave of the HPN mode only indicates whether the received call is a voice service or a short message service. Hence, if the radio wave of the HPN mode is received, the satellite communication terminal 60 informs a subscriber through a liquid crystal display (LCD) that a call of the voice service or short message service is received and generates an alert tone. Consequently, in the HPN mode, a satellite communication subscriber may be made aware of the fact that a call is received, but the subscriber cannot answer the call or receive a short message.